1. Field of the Invention
The present invention relates to a method for manufacturing a flexible semiconductor device.
Note that in this specification, the term “semiconductor device” refers to general devices which can function by utilizing semiconductor properties, and display devices using liquid crystal, electroluminescence (EL), or the like, semiconductor circuits, and electronic devices are all defined as semiconductor devices.
2. Description of the Related Art
In recent years, techniques for manufacturing semiconductor devices have remarkably developed. In particular, techniques for making semiconductor devices thin and flexible have been attracting attention.
As a method for manufacturing a flexible semiconductor device, a technique in which after a semiconductor element such as a thin film transistor (TFT) is formed over a base material such as a glass substrate or a quartz substrate, the semiconductor element is transferred from the base material to another base material (for example, a flexible base material) has developed. In order to transfer the semiconductor element to another base material, a step for separating the semiconductor element from the base material that is used in forming the semiconductor element is necessary.
Examples of methods for making semiconductor devices, particularly, light-emitting devices flexible in a separation step are given below.
The first method is a method in which an EL layer is formed after separation and transfer. For example, a resin for assisting in separation, which functions as a support, is applied to a top surface of a TFT substrate provided with a separation layer. Next, a trigger for separation is given to separate a substrate which is used in forming a TFT. After that, a flexible substrate is attached to a separation surface and the resin for assisting in separation is removed. Then, an EL layer is formed above the TFT and sealing is conducted.
The second method is a method in which an electrode, a partition wall, an EL layer, and the like are formed over a separation surface after separation. For example, a resin for assisting in separation, which functions as a support, is applied to a top surface of a TFT substrate provided with a separation layer. Next, a trigger for separation is given to separates a substrate which is used in forming a TFT. Then, a contact hole is formed on a separation surface, and indium oxide-tin oxide (ITO: indium tin oxide) is formed so as to be electrically connected to a drain of the TFT and patterned to form an electrode. After that, a partition wall and an EL layer are formed and a film and the like are attached for sealing. With the use of this method, light-emitting devices in which the film is attached to a bottom surface and the resin for assisting in separation is formed on a top surface can be obtained.
The third method is a method in which after an electrode, an EL layer, a resin for assisting in separation, and the like are formed, separation is conducted.
In the first method, steps for applying and removing the resin for assisting in separation are necessary. Thus, the number of steps is increased. In the second method, at least three photolithography steps (formation of the contact hole, patterning of the ITO, and formation of the partition wall) are necessary. Thus, the technical hurdle is high. Therefore, the number of steps in the third method is smaller than those in the first method and the second method, and it can be said that the third method is suitable for mass production.
As a method for separating the semiconductor element typified by a light-emitting element from the base material that is used in forming the semiconductor element, there is a method in which a separation layer is formed and a semiconductor element is separated along the separation layer. First, a separation layer is formed over a base material, and a semiconductor element film is formed over the separation layer. After that, the semiconductor element film is separated along the separation layer by applying physical force. Separation is conducted in such a manner, and the semiconductor element is made flexible.
For example, Patent Document 1 discloses a separation technique using laser ablation, which is described below. A separation layer is formed using amorphous silicon or the like over a substrate, a layer to be separated is formed using a thin film element over the separation layer, and the layer to be separated is bonded to a transfer object by using a bonding layer. The separation layer is ablated by laser irradiation, so that separation is generated in the separation layer.
In addition, Patent Document 2 discloses a technique in which separation is conducted by physical force such as human hands. In Patent Document 2, a metal layer is formed between a substrate and an oxide layer and separation is generated at an interface between the oxide layer and the metal layer by utilizing weak bonding between the oxide layer and the metal layer at the interface, so that the layer to be separated and the substrate are separated from each other.
In the case where separation is conducted by physical force such as human hands, the layer to be separated needs to be curved in order to separate the layer to be separated from the base material along the separation layer. The layer to be separated formed in contact with the separation layer is a very fragile thin film with a thickness of about 10 μm in which semiconductor elements including a thin film transistor (TFT), a wiring, an interlayer film, and the like are formed. When the bending stress is applied to the semiconductor elements, the layer to be separated is easily broken and cracked, which causes malfunctions such as breaking of the semiconductor elements frequently.
When separation is conducted, a portion where adhesion between the separation layer and the layer to be separated which are formed over the base material is weakest is preferentially separated. Therefore, adhesion at an interface between the base material and the separation layer or an interface between the separation layer and the layer to be separated needs to be weakest in a stacked-layer body including from the base material to the layer to be separated in order to start separation at the interface between the base material and the separation layer, at the interface between the separation layer and the layer to be separated, or inside the separation layer without separating a stacked film included in the layer to be separated.
Further, when the separation layer is a stacked film, separation can be conducted at the interface between the base material and the separation layer, at the interface between the separation layer and the layer to be separated, or inside the separation layer even if adhesion at interfaces between films included in the separation layer is weakest.
If adhesion between the base material and the separation layer, adhesion between the separation layer and the layer to be separated, or adhesion between films included in the separation layer is too weak, separation might be generated due to the stress of the film in the step in which separation is not expected to be conducted (in a step other than the separation step). Therefore, a process in which the separation layer keeps adhesion to some degree until just before the separation step and adhesion of the separation layer is intentionally reduced by conducting some sort of process in the separation step is preferable.
[Citation List]
[Patent Document 1] Japanese Published Patent Application No. H10-125931
[Patent Document 2] Japanese Published Patent Application No. 2003-174153